What are the signs and symptoms of an ACL injury?

A twist or strain has occurred which causes the following signs:

"Pop" - Many patients, but not all, will hear or feel a "pop" when the ACL tears.
Immediate onset of swelling - This is an indication that there is bleeding from the injured ligament.
Pain - Most patients experience quite a bit of pain with an ACL injury.
Instability - Patients often describe a buckling or unstable sensation in the knee.